1. Field of the Invention
The present invention generally relates to optical disk storage devices and, more particularly, to track following and gray-code seek patterns for sector servos on optical disks.
2. Description of the Prior Art
Optical disks achieve high a real density of information storage by using optical methods of track-following to get high track densities. This requires some method of incorporating servo information on the disk, in the manufacturing process, for the track-following servo to sense and follow. A variety of techniques can be used, such as etching of the substrate or a photopolymer process, to generate structures of varying depths that can carry servo information. Commonly used servos employ grooves, which act as diffraction gratings, as the servo structure. The width of the groove, plus the width of the intervening land, is then equal to the track pitch. Photolithiographic processes could be used to define the groove structures, but the capability of those processes to generate either fine grooves or fine spaces between them is a severe limitation on attainable track densities. An alternate approach, called biplanar, uses both the groove and the land at widths equal to the track pitch. This somewhat alleviates the problem, but it is only a partial solution and introduces other problems. What is needed is a method of serving, wherein the features (and the spaces between) used for servo information can be larger than the track pitch. The larger they can be made, the greater the potential reduction in track pitch that can be achieved using photolithographic reproduction. This is important as shorter wavelength lasers become available in the future.
When a track-seek operation is performed on an optical disk, it is highly desirable to be able to identify with considerable accuracy the exact location of the spot relative to the numbered tracks. There are two specific uses for this information; one is to enable accurate determination of the velocity of the head motion, and the second is to be able to monitor the approach to the desired track, adjust the velocity accordingly, and land on the desired track with a minimum of elapsed time and without error. When servo grooves are used to seek on disks, it is common practice to count the grooves as they are passed. When sector servos are used, however, tracks can be crossed between occurrences of sector-servo information. Although the sector-servo data allows the determination of spot location within the tracking-servo period, which can be three or four tracks, that resolution might not be adequate when very high speed seek motions are possible. For this reason, it is desirable to insert additional, radially coded patterns that will provide exact determination of spot position over a wider range of track numbers. Note that there is no need to encode a unique identifier for each track on the disk. Identification within a range of 30 to 50 tracks is adequate, as the range of uncertainty will not be greater. Encoding to obtain more resolution than is needed is wasteful as area used for codes cannot be used for data storage. Efficiency of disk space utilization, as in the design of the sector tracking servo patterns, is an essential requirement.
Because the useful data is written on top of the grooves, these are a significant source of noise that contributes to the data error rate and is a limitation of a real density and data rate. The required bandwidth of the tracking servos is fairly low. A full time servo signal is not required for accurate track following; therefore, a sectored servo method, which inserts servo information in a number of discrete locations in sectors around the track, is feasible. The sector-servo technique is quite well known in conventional magnetic disk recording. In known optical disk sector servo systems, the marks comprising the servo data have been made with feature widths and spacings approximately equal to the track pitch, arranged in several rows radially, to achieve a two-phase pattern. A first problem with this approach is the limited track pitch that can be achieved with photolithographic methods. While this might not be a problem with read-only optical disks that are fabricated by plastic molding from masters that can be made with short wavelength lasers, it is still a severe problem when molded grooves are not acceptable. A second problem with using small feature dimensions and separations, comparable to the reading laser spot diameter, is the poor optical resolution that is obtained. The signal outputs are therefore extremely variable relative to spot size and to the inevitable variations in feature sizes and separations caused by variability of the fabrication processes. The poor resolution also results in low signal amplitudes and sensitivity to noise interferences.